The Extensible Markup Language (XML) is a subset of SGML that is completely
described in this document. Its goal is to enable generic SGML to be served,
received, and processed on the Web in the way that is now possible with HTML.
XML has been designed for ease of implementation and for interoperability
with both SGML and HTML.

This section describes the status of this document at the time of its publication.
Other documents may supersede this document. A list of current W3C publications and the
latest revision of this technical report can be found in the W3C technical reports index at
http://www.w3.org/TR/.

This document specifies a syntax created by subsetting an existing, widely
used international text processing standard (Standard Generalized Markup Language,
ISO 8879:1986(E) as amended and corrected) for use on the World Wide Web.
It is a product of the XML Core Working Group
as part of the XML Activity.

On 29 September 2006 this document was edited in place to remove a number of spurious and potentially misleading spaces.

The English version of this specification is the only normative version. However,
for translations of this document, see &translationloc;.

This document is a W3C Recommendation.
This second edition is not a new version of XML. As a convenience to readers,
it incorporates the changes dictated by the accumulated errata (available at
&preverrataloc;) to the First
Edition of XML 1.1, dated 4 February 2004. In addition, the markup introduced to clarify when prescriptive
keywords are used
in the formal sense defined in , has been modified to better match the intent of .
This edition supersedes the previous W3C Recommendation
of 4 February 2004.

Please report errors in this document to the public xml-editor@w3.org mailing list; archives are available. For the convenience of readers,
an XHTML version with color-coded revision indicators is
also provided; this version highlights each change due to an erratum published in the
errata list, together with a link to the particular
erratum in that list. Most of the errata in the list provide a rationale for the change.
The errata list for this second edition is available at &errataloc;.

An implementation report is available at &impreploc;. A Test Suite is maintained to help assessing conformance to this specification.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

This document is governed by the 24
January 2002 CPP as amended by the W3C Patent Policy
Transition Procedure. W3C maintains a public list of
any patent disclosures made in connection with the deliverables of
the group; that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual
believes contains Essential
Claim(s) must disclose the information in accordance with section
6 of the W3C Patent Policy.

Extensible Markup Language, abbreviated XML, describes a class of data
objects called XML documents and partially
describes the behavior of computer programs which process them. XML is an
application profile or restricted form of SGML, the Standard Generalized Markup
Language . By construction, XML documents are conforming
SGML documents.

XML documents are made up of storage units called entities,
which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character
data, and some of which form markup.
Markup encodes a description of the document's storage layout and logical
structure. XML provides a mechanism to impose constraints on the storage layout
and logical structure.

A software module called
an XML processor is used to read XML documents and provide access
to their content and structure.It
is assumed that an XML processor is doing its work on behalf of another module,
called the application. This specification describes
the required behavior of an XML processor in terms of how it must read XML
data and the information it must provide to the application.

Origin and Goals

XML was developed by an XML Working Group (originally known as the SGML
Editorial Review Board) formed under the auspices of the World Wide Web Consortium
(W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with the active
participation of an XML Special Interest Group (previously known as the SGML
Working Group) also organized by the W3C. The membership of the XML Working
Group is given in an appendix. Dan Connolly served as the Working Group's contact with
the W3C.

The design goals for XML are:

XML shall be straightforwardly usable over the Internet.

XML shall support a wide variety of applications.

XML shall be compatible with SGML.

It shall be easy to write programs which process XML documents.

The number of optional features in XML is to be kept to the absolute
minimum, ideally zero.

XML documents should be human-legible and reasonably clear.

The XML design should be prepared quickly.

The design of XML shall be formal and concise.

XML documents shall be easy to create.

Terseness in XML markup is of minimal importance.

This specification, together with associated standards (Unicode
and ISO/IEC 10646
for characters, Internet RFC 3066 for
language identification tags, ISO 639
for language name codes, and ISO 3166 for
country name codes), provides all the information necessary to
understand XML Version &versionOfXML; and construct computer
programs to process it.

This version of the XML specification may be distributed freely, as long as
all text and legal notices remain intact.

Terminology

The terminology used to describe XML documents is defined in the body of
this specification. The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, &MAY;, and OPTIONAL, when EMPHASIZED, are to be interpreted as described in . In addition, the terms defined in the following list are used in building
those definitions and in describing the actions of an XML processor:error

A violation of the rules of this specification;
results are undefined. Unless otherwise specified, failure to observe a prescription of this specification indicated by one of the keywords MUST, REQUIRED, MUST NOT, SHALL and SHALL NOT is an error. Conforming software &MAY; detect and report an error
and &MAY; recover from it.

fatal error

An error which a conforming XML processorMUST detect and report to the application.
After encountering a fatal error, the processor &MAY; continue processing the
data to search for further errors and &MAY; report such errors to the application.
In order to support correction of errors, the processor &MAY; make unprocessed
data from the document (with intermingled character data and markup) available
to the application. Once a fatal error is detected, however, the processor
MUST NOT continue normal processing (i.e., it MUST NOT continue to pass character
data and information about the document's logical structure to the application
in the normal way).

at user option

Conforming software
&MAY; or MUST (depending on the modal verb in the sentence) behave as described;
if it does, it MUST provide users a means to enable or disable the behavior
described.

validity constraint

A rule which applies to
all valid XML documents. Violations of validity
constraints are errors; they MUST, at user option, be reported by validating XML processors.

well-formedness constraint

A rule which applies
to all well-formed XML documents. Violations
of well-formedness constraints are fatal errors.

match

(Of strings or names:) Two strings
or names being compared MUST
beare identical. Characters with multiple possible
representations in Unicode (e.g. characters with both precomposed and
base+diacritic forms) match only if they have the same representation in both
strings. No
case folding is performed. (Of strings and rules in the grammar:) A string
matches a grammatical production if it belongs to the language generated by
that production. (Of content and content models:) An element matches its declaration
when it conforms in the fashion described in the constraint .

for compatibility

Marks
a sentence describing a feature of XML included solely to ensure
that XML remains compatible with SGML.

for interoperability

Marks
a sentence describing a non-binding recommendation included to increase
the chances that XML documents can be processed by the existing installed
base of SGML processors which predate the &WebSGML;.

Rationale and list of changes for XML 1.1

The W3C's XML 1.0 Recommendation was first issued in 1998, and
despite the issuance of many errata culminating in a Third Edition
of 2004, has remained (by intention) unchanged with respect to what
is well-formed XML and what is not. This stability has been
extremely useful for interoperability. However, the Unicode
Standard on which XML 1.0 relies for character specifications has
not remained static, evolving from version 2.0 to version 4.0 and
beyond. Characters not present in Unicode 2.0 may already be used
in XML 1.0 character data. However, they are not allowed in XML
names such as element type names, attribute names, enumerated
attribute values, processing instruction targets, and so on. In
addition, some characters that should have been permitted in XML
names were not, due to oversights and inconsistencies in Unicode
2.0.

The overall philosophy of names has changed since XML 1.0.
Whereas XML 1.0 provided a rigid definition of names, wherein
everything that was not permitted was forbidden, XML 1.1 names are
designed so that everything that is not forbidden (for a specific
reason) is permitted. Since Unicode will continue to grow past
version 4.0, further changes to XML can be avoided by allowing
almost any character, including those not yet assigned, in
names.

In addition, XML 1.0 attempts to adapt to the line-end
conventions of various modern operating systems, but discriminates
against the conventions used on IBM and IBM-compatible mainframes.
As a result, XML documents on mainframes are not plain text files
according to the local conventions. XML 1.0 documents generated on
mainframes must either violate the local line-end conventions, or
employ otherwise unnecessary translation phases before parsing and
after generation. Allowing straightforward interoperability is
particularly important when data stores are shared between
mainframe and non-mainframe systems (as opposed to being copied
from one to the other). Therefore XML 1.1 adds NEL (#x85) to the
list of line-end characters. For completeness, the Unicode line
separator character, #x2028, is also supported.

Finally, there is considerable demand to define a standard representation
of arbitrary Unicode characters in XML documents. Therefore, XML 1.1
allows the use of character references to the control characters #x1 through
#x1F, most of which are forbidden in XML 1.0. For reasons of robustness,
however, these characters still cannot be used directly in documents. In
order to improve the robustness of character encoding detection, the additional
control characters #x7F through #x9F, which were freely allowed in XML 1.0
documents, now must also appear only as character references. (Whitespace
characters are of course exempt.) The minor sacrifice of backward compatibility
is considered not significant. Due to potential problems with APIs,
#x0 is still forbidden both directly and as a character reference.

Finally, XML 1.1 defines a set of constraints called "full
normalization" on XML documents, which document creators
SHOULD adhere to, and document processors
SHOULD verify. Using fully normalized documents
ensures that identity comparisons of names, attribute values, and
character content can be made correctly by simple binary comparison of
Unicode strings.

A new XML version, rather than a set of errata to XML 1.0, is
being created because the changes affect the definition of
well-formed documents. XML 1.0 processors must continue to reject
documents that contain new characters in XML names, new line-end
conventions, and references to control characters. The distinction between XML 1.0 and XML 1.1 documents
is indicated by the version number information in the XML
declaration at the start of each document.

Documents

A data object is an XML
document if it is well-formed,
as defined in this specification. In addition, the XML document isvalid if it meets certain further constraints.

Each XML document has both a logical and a physical structure. Physically,
the document is composed of units called entities.
An entity &may; refer to other entities to
cause their inclusion in the document. A document begins in a root
or document entity. Logically, the document
is composed of declarations, elements, comments, character references, and
processing instructions, all of which are indicated in the document by explicit
markup. The logical and physical structures MUST nest properly, as described
in .

Well-Formed XML Documents

A textual object is a well-formed
XML document if:

Taken as a whole, it matches the production labeled document.

It meets all the well-formedness constraints given in this specification.

Each of the parsed entities
which is referenced directly or indirectly within the document is well-formed.

Document
document(prologelementMisc*)- (Char* RestrictedCharChar*)

Matching the document production implies that:

It contains one or more elements.

There is exactly one element,
called the root, or document element, no part of which appears
in the content of any other element. For
all other elements, if the start-tag is in
the content of another element, the end-tag
is in the content of the same element. More simply stated, the elements,
delimited by start- and end-tags, nest properly within each other.

As a consequence of this,
for each non-root element C in the document, there is one other element P
in the document such that C is in the content of P, but
is not in the content of any other element that is in the content of P. P
is referred to as the parent of C, and C as
a child of P.

Characters

A parsed entity contains text,
a sequence of characters, which may
represent markup or character data.A character
is an atomic unit of text as specified by ISO/IEC 10646 . Legal characters are tab, carriage
return, line feed, and the legal characters
of Unicode and ISO/IEC 10646. The
versions of these standards cited in were
current at the time this document was prepared. New characters may be added
to these standards by amendments or new editions. Consequently, XML processors
MUST accept any character in the range specified for Char.

The mechanism for encoding character code points into bit patterns &may;
vary from entity to entity. All XML processors MUST accept the UTF-8 and UTF-16
encodings of Unicode ;
the mechanisms for signaling which of the two is in use,
or for bringing other encodings into play, are discussed later, in .

Document authors are encouraged to avoid
compatibility characters, as defined
in Unicode .
The characters defined in the following ranges are also
discouraged. They are either control characters or permanently undefined Unicode
characters:

S (white space) consists of one or more space (#x20)
characters, carriage returns, line feeds, or tabs.

White Space
S(#x20 | #x9 | #xD | #xA)+

The presence of #xD in the above production is
maintained purely for backward compatibility with the
First Edition.
As explained in ,
all #xD characters literally present in an XML document
are either removed or replaced by #xA characters before
any other processing is done. The only way to get a #xD character to match this production is to
use a character reference in an entity value literal.

A Name is a token beginning
with a letter or one of a few punctuation characters, and continuing with
letters, digits, hyphens, underscores, colons, or full stops, together known
as name characters. Names beginning with the string xml,
or with any string which would match (('X'|'x') ('M'|'m') ('L'|'l')),
are reserved for standardization in this or future versions of this specification.

The
Namespaces in XML Recommendation assigns a meaning
to names containing colon characters. Therefore, authors should not use the
colon in XML names except for namespace purposes, but XML processors must
accept the colon as a name character.

An Nmtoken (name token) is any mixture of name
characters.

The first character of a Name MUST be a NameStartChar, and any
other characters MUST be NameChars; this mechanism is used to
prevent names from beginning with European (ASCII) digits or with
basic combining characters. Almost all characters are permitted in
names, except those which either are or reasonably could be used as
delimiters. The intention is to be inclusive rather than exclusive,
so that writing systems not yet encoded in Unicode can be used in
XML names. See for suggestions on the creation of
names.

Document authors are encouraged to use names which are
meaningful words or combinations of words in natural languages, and
to avoid symbolic or white space characters in names. Note that
COLON, HYPHEN-MINUS, FULL STOP (period), LOW LINE (underscore), and
MIDDLE DOT are explicitly permitted.

The ASCII symbols and punctuation marks, along with a fairly
large group of Unicode symbol characters, are excluded from names
because they are more useful as delimiters in contexts where XML
names are used outside XML documents; providing this group gives
those contexts hard guarantees about what cannot be part of
an XML name. The character #x037E, GREEK QUESTION MARK, is excluded
because when normalized it becomes a semicolon, which could change
the meaning of entity references.

The Names
and Nmtokens productions are used to define the validity
of tokenized attribute values after normalization (see ).

Literal data is any quoted string not containing the quotation mark used
as a delimiter for that string. Literals are used for specifying the content
of internal entities (EntityValue), the values
of attributes (AttValue), and external identifiers
(SystemLiteral). Note that a SystemLiteral
can be parsed without scanning for markup.

Although
the EntityValue production allows the definition
of a general entity consisting of a single explicit < in the literal
(e.g., <!ENTITY mylt "<">), it is strongly advised to avoid
this practice since any reference to that entity will cause a well-formedness
error.

Character Data and Markup

Text consists of intermingled character data and markup. Markup takes the form of start-tags, end-tags, empty-element tags, entity references, character
references, comments, CDATA section delimiters, document
type declarations, processing instructions, XML declarations, text declarations,
and any white space that is at the top level of the document entity (that
is, outside the document element and not inside any other markup).

All text that is not markup
constitutes the character data of the document.

The ampersand character (&) and the left angle bracket (<) MUST NOT appear
in their literal form, except when used as markup delimiters, or
within a comment, a processing
instruction, or a CDATA section.
If they are needed elsewhere, they MUST be escaped
using either numeric character references
or the strings &amp; and &lt;
respectively. The right angle bracket (>) &may; be represented using the string &gt;,
and MUST, for compatibility, be escaped
using either &gt; or a character reference when it
appears in the string ]]> in content, when
that string is not marking the end of a CDATA
section.

In the content of elements, character data is any string of characters
which does not contain the start-delimiter of any markup or the
CDATA-section-close delimiter,
]]>.
In a CDATA section,
character data is any string of characters not including the CDATA-section-close
delimiter.

To allow attribute values to contain both single and double quotes, the
apostrophe or single-quote character (') &may; be represented as &apos;,
and the double-quote character (") as &quot;.

Character Data
CharData[^<&]* - ([^<&]* ']]>' [^<&]*)
Comments

Comments &may; appear
anywhere in a document outside other markup;
in addition, they &may; appear within the document type declaration at places
allowed by the grammar. They are not part of the document's character
data; an XML processor &MAY;, but need not, make it possible for an
application to retrieve the text of comments. For
compatibility, the string -- (double-hyphen)
MUST NOT occur within comments. Parameter
entity references MUST NOT be recognized within comments.

Comments
Comment'<!--' ((Char - '-') | ('-'
(Char - '-')))* '-->'

An example of a comment:

<!-- declarations for <head> & <body> -->

Note
that the grammar does not allow a comment ending in --->. The
following example is not well-formed.

PIs are not part of the document's character
data, but MUST be passed through to the application. The PI begins
with a target (PITarget) used to identify the application
to which the instruction is directed. The target names XML, xml,
and so on are reserved for standardization in this or future versions of this
specification. The XML Notation mechanism
&may; be used for formal declaration of PI targets. Parameter
entity references MUST NOT be recognized within processing instructions.

CDATA Sections

CDATA sections &may; occur anywhere character data may occur; they are used to escape blocks
of text containing characters which would otherwise be recognized as markup.
CDATA sections begin with the string <![CDATA[
and end with the string ]]>:

Within a CDATA section, only the CDEnd string is
recognized as markup, so that left angle brackets and ampersands may occur
in their literal form; they need not (and cannot) be escaped using &lt;
and &amp;. CDATA sections cannot nest.

An example of a CDATA section, in which <greeting>
and </greeting> are recognized as character data, not markup:

XML 1.1 documents MUST
begin with an XML declaration which specifies the version of
XML being used. For example, the following is a complete XML 1.1 document, well-formed but not valid:

Hello, world! ]]>

but the following is an XML 1.0 document because it
does not have an XML declaration:

Hello, world!]]>

The function of the markup in an XML document is to describe its storage and
logical structure and to associate attribute
name-value pairs with its logical structures. XML provides a mechanism, the
document
type declaration, to define constraints on the logical structure
and to support the use of predefined storage units. An XML document is valid if it has an associated
document type declaration and if the document complies with the constraints
expressed in it.

The document type declaration MUST appear before the first element
in the document.

The XML document
type declaration contains or points to markup
declarations that provide a grammar for a class of documents. This
grammar is known as a document type definition, or DTD. The document
type declaration can point to an external subset (a special kind of external entity) containing markup declarations,
or can contain the markup declarations directly in an internal subset, or
can do both. The DTD for a document consists of both subsets taken together.

A markup declaration
is an element type declaration, an attribute-list declaration, an entity
declaration, or a notation declaration.
These declarations &may; be contained in whole or in part within parameter
entities, as described in the well-formedness and validity constraints
below. For further
information, see .

Note
that it is possible to construct a well-formed document containing a doctypedecl
that neither points to an external subset nor contains an internal subset.

The markup declarations &may; be made up in whole or in part of the replacement text of parameter
entities. The productions later in this specification for individual
nonterminals (elementdecl, AttlistDecl,
and so on) describe the declarations after all the parameter
entities have been included.

Parameter
entity references are recognized anywhere in the DTD (internal and external
subsets and external parameter entities), except in literals, processing instructions,
comments, and the contents of ignored conditional sections (see ).
They are also recognized in entity value literals. The use of parameter entities
in the internal subset is restricted as described below.

Root Element Type

The Name
in the document type declaration MUST match the element type of the root element.

Proper Declaration/PE Nesting

Parameter-entity replacement textMUST be properly nested with markup declarations. That is to say, if either
the first character or the last character of a markup declaration (markupdecl
above) is contained in the replacement text for a parameter-entity
reference, both MUST be contained in the same replacement text.

PEs in Internal Subset

In
the internal DTD subset, parameter-entity referencesMUST NOT occur within markup declarations; they &may; occur where markup declarations can occur.
(This does not apply to references that occur in external parameter entities
or to the external subset.)

External Subset

The external subset, if any, MUST match the production for extSubset.

PE Between Declarations

The replacement text of a parameter entity reference
in a DeclSepMUST match the production extSubsetDecl.

Like the internal subset, the external subset and any external parameter
entities referenced
in a DeclSepMUST consist of a series of
complete markup declarations of the types allowed by the non-terminal symbol markupdecl, interspersed with white space or parameter-entity references. However, portions of
the contents of the external subset or of these
external parameter entities &may; conditionally be ignored by using the conditional section construct; this is not
allowed in the internal subset but is
allowed in external parameter entities referenced in the internal subset.

The external subset and external parameter entities also differ from the
internal subset in that in them, parameter-entity
references are permitted within markup declarations,
not only between markup declarations.

An example of an XML document with a document type declaration:

Hello, world! ]]>

The system identifierhello.dtd
gives the address (a URI reference) of a DTD for the document.

The declarations can also be given locally, as in this example:

]>
Hello, world!]]>

If both the external and internal subsets are used, the internal subset
MUST be considered to occur before the external subset.
This has the effect that entity and attribute-list declarations in the internal
subset take precedence over those in the external subset.

XML 1.1 processors SHOULD accept XML 1.0
documents as well. If a document is well-formed or valid XML 1.0, and provided it
does not contain any control characters
in the range [#x7F-#x9F] other than as character escapes, it may be
made well-formed or valid XML 1.1 respectively simply by changing the
version number.

Standalone Document Declaration

Markup declarations can affect the content of the document, as passed from
an XML processor to an application; examples
are attribute defaults and entity declarations. The standalone document declaration,
which &may; appear as a component of the XML declaration, signals whether or
not there are such declarations which appear external to the document
entity
or in parameter entities. An external
markup declaration is defined as a markup declaration occurring in
the external subset or in a parameter entity (external or internal, the latter
being included because non-validating processors are not required to read
them).

In a standalone document declaration, the value yes indicates
that there are no external markup declarations which
affect the information passed from the XML processor to the application. The
value no indicates that there are or may be such external
markup declarations. Note that the standalone document declaration only denotes
the presence of external declarations; the presence, in a document,
of references to external entities, when those entities are internally
declared, does not change its standalone status.

If there are no external markup declarations, the standalone document declaration
has no meaning. If there are external markup declarations but there is no
standalone document declaration, the value no is assumed.

Any XML document for which standalone="no" holds can be converted
algorithmically to a standalone document, which may be desirable for some
network delivery applications.

Standalone Document Declaration

The
standalone document declaration MUST have the value no if
any external markup declarations contain declarations of:

attributes with default values,
if elements to which these attributes apply appear in the document without
specifications of values for these attributes, or

entities (other than &magicents;), if references
to those entities appear in the document, or

attributes with
tokenized types, where the
attribute appears in the document with a value such that
normalization
will produce a different value from that which would be produced
in the absence of the declaration, or

element types with element content,
if white space occurs directly within any instance of those types.

In editing XML documents, it is often convenient to use white space
(spaces, tabs, and blank lines)
to set apart the markup for greater readability. Such white space is typically
not intended for inclusion in the delivered version of the document. On the
other hand, significant white space that should be preserved
in the delivered version is common, for example in poetry and source code.

An XML processorMUST always pass
all characters in a document that are not markup through to the application.
A validating XML processorMUST also
inform the application which of these characters constitute white space appearing
in element content.

A special attribute named xml:space &may; be attached to an element to signal an intention that in that element,
white space should be preserved by applications. In valid documents, this
attribute, like any other, MUST be declared
if it is used. When declared, it MUST be given as an enumerated
type whose values
are one or both of default and preserve.
For example:

]]>
<!ATTLIST pre xml:space (preserve) #FIXED 'preserve'>

The value default signals that applications' default white-space
processing modes are acceptable for this element; the value preserve
indicates the intent that applications preserve all the white space. This
declared intent is considered to apply to all elements within the content
of the element where it is specified, unless overridden with
another instance of the xml:space attribute. This specification does not give meaning to any value of xml:space other than default and preserve. It is an error for other values to be specified; the XML processor &MAY; report the error or &MAY; recover by ignoring the attribute specification or by reporting the (erroneous) value to the application. Applications may ignore or reject erroneous values.

The root element of any document is considered
to have signaled no intentions as regards application space handling, unless
it provides a value for this attribute or the attribute is declared with a
default value.

End-of-Line Handling

XML parsed entities are often stored
in computer files which, for editing convenience, are organized into lines.
These lines are typically separated by some combination of the characters
CARRIAGE RETURN (#xD) and LINE FEED (#xA).

To
simplify the tasks of applications, the
XML
processorMUST behave as if it normalized all line breaks in external parsed
entities (including the document entity) on input, before parsing, by translating
all of the following to a single #xA character:

the two-character sequence #xD #xA

the two-character sequence #xD #x85

the single character #x85

the single character #x2028

any #xD character that is not immediately followed by #xA or #x85.

The characters #x85 and #x2028 cannot be reliably recognized and
translated until an entity's encoding declaration (if present) has
been read. Therefore, it is a fatal error to use them within the XML
declaration or text declaration.

Language Identification

In document processing, it is often useful to identify the natural or formal
language in which the content is written. A special attribute
named xml:lang &may; be inserted in documents to specify the language
used in the contents and attribute values of any element in an XML document.
In valid documents, this attribute, like any other, MUST be declared
if it is used. The
values of the attribute are language identifiers as defined by , Tags
for the Identification of Languages, or its successor; in addition, the empty string &may; be specified.

The language specified byxml:langis considered to applies
to all attributes and content of the element where it is specified
(including the values of its attributes), and to all elements in its content unless
overridden with another instance of xml:lang on another element within
that content. In particular, the empty value of xml:lang is used on an element B to override
a specification of xml:lang on an enclosing element A, without specifying another language. Within B,
it is considered that there is no language information available, just as if xml:lang had not been specified
on B or any of its ancestors. Applications determine which of an element's attribute values
and which parts of its character content, if any, are treated as language-dependent values described by xml:lang.

Language information may also be provided by external transport protocols (e.g. HTTP or
MIME). When available, this information may be used by XML applications, but the more local
information provided by xml:lang should be considered to override it.

A simple declaration for xml:lang might take the form

xml:lang CDATA #IMPLIED

but specific default values &may; also be given, if appropriate. In a collection
of French poems for English students, with glosses and notes in English, the xml:lang
attribute might be declared this way:

All XML parsed
entities (including document
entities) SHOULD be fully
normalized as per the definition of
supplemented by the following definitions of
relevant constructs for XML:

The
replacement text of all parsed
entities

All text matching, in context, one of the following
productions:

CData

CharData

content

Name

Nmtoken

However, a document is still well-formed even if it is not
fully normalized.
XML processors SHOULD provide a user option to verify that the document being
processed is in fully normalized form, and report to the application whether
it is or not. The option to not verify SHOULD be chosen only when the
input text is certified,
as defined by .

The verification of full normalization MUST be carried out as if by
first verifying that the entity is in include-normalized
form as defined by and by then verifying that none of the relevant
constructs listed above begins (after character references are
expanded) with a composing character as defined by
.
Non-validating processors MUST ignore possible
denormalizations that would be caused by inclusion of external
entities that they do not read.

The composing character are all
Unicode characters of non-zero combining class, plus a small number
of class-zero characters that nevertheless take part as a
non-initial character in certain Unicode canonical
decompositions. Since these characters are meant to follow
base characters, restricting relevant constructs (including
content) from beginning with a composing character does not
meaningfully diminish the expressiveness of XML.

If, while verifying full normalization, a processor encounters
characters for which it cannot determine the normalization
properties (i.e., characters introduced in a version of Unicode
later than the one used in the implementation of the processor),
then the processor &MAY;, at user option, ignore any possible
denormalizations caused by these characters. The option to ignore
those denormalizations SHOULD NOT be chosen by applications when
reliability or security are critical.

XML processors MUST NOT transform the input to be in
fully normalized form.
XML applications that create XML 1.1 output
from either XML 1.1 or XML 1.0 input SHOULD ensure that the output
is fully normalized; it is not necessary for internal processing
forms to be fully normalized.

The purpose of this section is to strongly encourage XML
processors to ensure that the creators of XML documents have
properly normalized them, so that XML applications can make tests
such as identity comparisons of strings without having to worry
about the different possible "spellings" of strings which
Unicode allows.

When entities are in a non-Unicode encoding, if the processor
transcodes them to Unicode, it SHOULD use a normalizing transcoder.

Logical Structures

Each XML
document contains one or more elements, the boundaries
of which are either delimited by start-tags
and end-tags, or, for empty
elements, by an empty-element tag. Each
element has a type, identified by name, sometimes called its generic
identifier (GI), and &may; have a set of attribute specifications.
Each attribute specification has a name
and a value.

Element
elementEmptyElemTag| STagcontentETag

This specification does not constrain the
application semantics, use, or (beyond syntax)
names of the element types and attributes, except that names beginning with
a match to (('X'|'x')('M'|'m')('L'|'l')) are reserved for standardization
in this or future versions of this specification.

Element Type Match

The Name
in an element's end-tag MUST match the element type in the start-tag.

Element Valid

An element is valid
if there is a declaration matching elementdecl
where the Name matches the element type, and one of
the following holds:

The declaration matches EMPTY and the element has no content (not even entity
references, comments, PIs or white space).

The declaration matches children and the
sequence of child elements belongs
to the language generated by the regular expression in the content model,
with optional white space, comments and
PIs (i.e. markup matching production [27] Misc) between the
start-tag and the first child element, between child elements, or between
the last child element and the end-tag. Note that a CDATA section containing
only white space or a reference
to an entity whose replacement text is character references expanding to white
space do not
match the nonterminal S, and
hence cannot appear in these positions; however, a
reference to an internal entity with a literal value consisting of character
references expanding to white space does match S, since its
replacement text is the white space resulting from expansion of the character
references.

The declaration matches Mixed, and the content
(after replacing
any entity references with their replacement text) consists of
character data (including CDATA sections),
comments, PIs and child elements whose types match names in the
content model.

The declaration matches ANY, and the
content (after replacing
any entity references with their replacement text)
consists of character data, CDATA
sections, comments, PIs and child elements
whose types have been declared.

Start-Tags, End-Tags, and Empty-Element Tags

The beginning of every non-empty
XML element is marked by a start-tag.

Start-tag
STag'<' Name (SAttribute)* S? '>'AttributeNameEqAttValue

The Name in the start- and end-tags gives the element's type. The Name-AttValue
pairs are referred to as the attribute specifications of the
element, with the Name in each pair referred to as the attribute name
and the content of the AttValue (the text between the ' or "
delimiters) as the attribute value. Note
that the order of attribute specifications in a start-tag or empty-element
tag is not significant.

Unique Att Spec

An attribute name
MUST NOT appear more than once in the same start-tag or empty-element tag.

Attribute Value Type

The attribute MUST
have been declared; the value MUST be of the type declared for it. (For attribute
types, see .)

No External Entity References

Attribute
values MUST NOT contain direct or indirect entity references to external entities.

No < in Attribute Values

The replacement text of any entity
referred to directly or indirectly in an attribute value MUST NOT contain a <.

An example of a start-tag:

<termdef id="dt-dog" term="dog">

The end of every element that begins
with a start-tag MUST be marked by an end-tag containing a name
that echoes the element's type as given in the start-tag:

End-tag
ETag'</' NameS?
'>'

An example of an end-tag:

</termdef>

The text
between the start-tag and end-tag is called the element's content:

An element
with no content is said to be empty. The representation
of an empty element is either a start-tag immediately followed by an end-tag,
or an empty-element tag. An empty-element
tag takes a special form:

Tags for Empty Elements
EmptyElemTag'<' Name (SAttribute)* S? '/>'

Empty-element tags &may; be used for any element which has no content, whether
or not it is declared using the keyword EMPTY. For
interoperability, the empty-element tag SHOULD
be used, and SHOULD only be used, for elements which are declared
EMPTY.

The element structure of an XML document &may;, for validation
purposes, be constrained using element type and attribute-list declarations.
An element type declaration constrains the element's content.

Element type declarations often constrain which element types can appear
as children of the element. At user
option, an XML processor &MAY; issue a warning when a declaration mentions an
element type for which no declaration is provided, but this is not an error.

An element type has element content when elements
of that type MUST contain only child
elements (no character data), optionally separated by white space (characters
matching the nonterminal S).In this case, the constraint includes a content
model, a simple grammar governing the allowed types of the
child elements and the order in which they are allowed to appear.
The grammar is built on content particles (cps), which
consist of names, choice lists of content particles, or sequence lists of
content particles:

where each Name is the type of an element which
&may; appear as a child. Any content
particle in a choice list &may; appear in the element
content at the location where the choice list appears in the grammar;
content particles occurring in a sequence list MUST each appear in the element content in the order given in the list.
The optional character following a name or list governs whether the element
or the content particles in the list may occur one or more (+),
zero or more (*), or zero or one times (?). The
absence of such an operator means that the element or content particle MUST
appear exactly once. This syntax and meaning are identical to those used in
the productions in this specification.

The content of an element matches a content model if and only if it is
possible to trace out a path through the content model, obeying the sequence,
choice, and repetition operators and matching each element in the content
against an element type in the content model. For
compatibility, it is an error if the content model
allows an element to match more than one occurrence of an element type in the
content model. For more information, see .

Proper Group/PE Nesting

Parameter-entity replacement textMUST be properly nested with parenthesized
groups. That is to say, if either of the opening or closing parentheses in
a choice, seq, or Mixed
construct is contained in the replacement text for a parameter
entity, both MUST be contained in the same replacement text.

For interoperability, if a parameter-entity reference
appears in a choice, seq, or Mixed construct, its replacement text SHOULD contain at
least one non-blank character, and neither the first nor last non-blank character
of the replacement text SHOULD be a connector (| or ,).

An element type
has mixed content when elements of that type &may; contain character
data, optionally interspersed with child
elements. In this case, the types of the child elements &may; be constrained,
but not their order or their number of occurrences:

Attributes are used to associate name-value
pairs with elements. Attribute specifications
MUST NOT appear outside of start-tags and empty-element tags; thus, the productions used to
recognize them appear in . Attribute-list declarations
&may; be used:

To define the set of attributes pertaining to a given element type.

To establish type constraints for these attributes.

To provide default values for
attributes.

Attribute-list
declarations specify the name, data type, and default value (if any)
of each attribute associated with a given element type:

The Name in the AttlistDecl
rule is the type of an element. At user option, an XML processor &MAY; issue
a warning if attributes are declared for an element type not itself declared,
but this is not an error. The Name in the AttDef
rule is the name of the attribute.

When more than one AttlistDecl is provided
for a given element type, the contents of all those provided are merged. When
more than one definition is provided for the same attribute of a given element
type, the first declaration is binding and later declarations are ignored. For interoperability, writers of DTDs &may; choose
to provide at most one attribute-list declaration for a given element type,
at most one attribute definition for a given attribute name in an attribute-list
declaration, and at least one attribute definition in each attribute-list
declaration. For interoperability, an XML processor &MAY; at user option
issue a warning when more than one attribute-list declaration is provided
for a given element type, or more than one attribute definition is provided
for a given attribute, but this is not an error.

Attribute Types

XML attribute types are of three kinds: a string type, a set of tokenized
types, and enumerated types. The string type may take any literal string as
a value; the tokenized types have varying lexical and semantic constraintsare more constrained.
The validity constraints noted in the grammar are applied after the attribute
value has been normalized as described in .

Values of type IDMUST match the Name production. A name MUST NOT appear more than once
in an XML document as a value of this type; i.e., ID values MUST uniquely
identify the elements which bear them.

One ID per Element Type

An element type MUST NOT have more than one ID attribute specified.

ID Attribute Default

An ID attribute
MUST have a declared default of #IMPLIED or #REQUIRED.

IDREF

Values of type IDREFMUST
match the Name production, and values of type IDREFSMUST match Names; each NameMUST match the value of an ID attribute on some element in the XML document;
i.e. IDREF values MUST match the value of some ID attribute.

Entity Name

Values of type ENTITYMUST match the Name production, values of type ENTITIESMUST match Names; each NameMUST match the name of an unparsed entity
declared in the DTD.

Enumerated attributesMUST take one of a list of values
provided in the declarationhave a list of allowed
values in their declaration. They MUST take one of those values. There are two kinds of enumerated attribute types:

In an attribute declaration, #REQUIRED means that the attribute
MUST always be provided, #IMPLIED that no default value is provided.
If
the declaration is neither #REQUIRED nor #IMPLIED, then
the AttValue value contains the declared default
value; the #FIXED keyword states that the attribute MUST always have
the default value.
When an XML processor encounters
an element
without a specification for an attribute for which it has read a default
value declaration, it MUST report the attribute with the declared default
value to the application.

Required Attribute

If the default
declaration is the keyword #REQUIRED, then the attribute MUST be
specified for all elements of the type in the attribute-list declaration.

Attribute Default Value Syntactically Correct

The declared default value MUST meet the syntactic
constraints of the declared attribute type. That is, the default value of an attribute:

of type IDREF or ENTITY must match the Name production;

of type IDREFS or ENTITIES must match the Names production;

of type NMTOKEN must match the Nmtoken production;

of type NMTOKENS must match the Nmtokens production;

of an enumerated type (either a NOTATION type or an enumeration) must match one of the enumerated values.

Note that only the
syntactic constraints of the type are required here; other constraints (e.g.
that the value be the name of a declared unparsed entity, for an attribute of
type ENTITY) may come into play if the declared default value is actually used
(an element without a specification for this attribute occurs)will be reported by a validating
parser only if an element without a specification for this attribute
actually occurs.

Fixed Attribute Default

If an attribute
has a default value declared with the #FIXED keyword, instances of
that attribute MUST match the default value.

Before the value of an attribute is passed to the application or checked
for validity, the XML processor MUST normalize the attribute value by applying
the algorithm below, or by using some other method such that the value passed
to the application is the same as that produced by the algorithm.

All line breaks MUST have been normalized on input to #xA as described
in , so the rest of this algorithm operates
on text normalized in this way.

Begin with a normalized value consisting of the empty string.

For each character, entity reference, or character reference in the
unnormalized attribute value, beginning with the first and continuing to the
last, do the following:

For a character reference, append the referenced character to the
normalized value.

For an entity reference, recursively apply step 3 of this algorithm
to the replacement text of the entity.

For a white space character (#x20, #xD, #xA, #x9), append a space
character (#x20) to the normalized value.

For another character, append the character to the normalized value.

If the attribute type is not CDATA, then the XML processor MUST further
process the normalized attribute value by discarding any leading and trailing
space (#x20) characters, and by replacing sequences of space (#x20) characters
by a single space (#x20) character.

Note that if the unnormalized attribute value contains a character reference
to a white space character other than space (#x20), the normalized value contains
the referenced character itself (#xD, #xA or #x9). This contrasts with the
case where the unnormalized value contains a white space character (not a
reference), which is replaced with a space character (#x20) in the normalized
value and also contrasts with the case where the unnormalized value contains
an entity reference whose replacement text contains a white space character;
being recursively processed, the white space character is replaced with a
space character (#x20) in the normalized value.

All attributes for which no declaration has been read SHOULD be treated
by a non-validating processor as if declared CDATA.

It
is an error if an
attribute
value contains a reference to an
entity for which no declaration has been read.

Following are examples of attribute normalization. Given the following
declarations:

<!ENTITY d "&#xD;">
<!ENTITY a "&#xA;">
<!ENTITY da "&#xD;&#xA;">

the attribute specifications in the left column below would be normalized
to the character sequences of the middle column if the attribute a
is declared NMTOKENS and to those of the right columns if a
is declared CDATA.

Attribute specification

a is NMTOKENS

a is CDATA

a="
xyz"

x y z

#x20 #x20 x y z

a="&d;&d;A&a;&#x20;&a;B&da;"

A #x20 B

#x20 #x20 A #x20 #x20 #x20 B #x20 #x20

a=
"&#xd;&#xd;A&#xa;&#xa;B&#xd;&#xa;"

#xD #xD A #xA #xA B #xD #xA

#xD #xD A #xA #xA B #xD #xA

Note that the last example is invalid (but well-formed) if a
is declared to be of type NMTOKENS.

Conditional Sections

Conditional
sections are portions of the document type
declaration external subset or
of external parameter entities which are included in, or excluded from,
the logical structure of the DTD based on the keyword which governs them.

If any of the "<![",
"[", or "]]>" of a conditional section is contained
in the replacement text for a parameter-entity reference, all of them MUST
be contained in the same replacement text.

Like the internal and external DTD subsets, a conditional section may contain
one or more complete declarations, comments, processing instructions, or nested
conditional sections, intermingled with white space.

If the keyword of the conditional section is INCLUDE, then the
contents of the conditional section MUST be consideredprocessed as
part of the DTD. If the keyword of
the conditional section is IGNORE, then the contents of the conditional
section MUSTbe
considered as not logicallyNOT be processed as part of the DTD.
If a conditional section with a keyword of INCLUDE occurs within
a larger conditional section with a keyword of IGNORE, both the outer
and the inner conditional sections MUST be ignored. The contents
of an ignored conditional section MUST be parsed by ignoring all characters after
the "[" following the keyword, except conditional section starts
"<![" and ends "]]>", until the matching conditional
section end is found. Parameter entity references MUST NOT be recognized in this
process.

If the keyword of the conditional section is a parameter-entity reference,
the parameter entity MUST be replaced by its content before the processor
decides whether to include or ignore the conditional section.

An XML document may consist of one
or many storage units. These
are called entities; they all have content and are
all (except for the document entity and
the external DTD subset) identified by
entity name. Each XML document has one entity
called the document entity, which serves
as the starting point for the XML processor
and may contain the whole document.

Entities may be either parsed or unparsed. The contents of a parsed
entity are referred to as its replacement
text; this text is considered an
integral part of the document.

An unparsed entity
is a resource whose contents may or may not be text,
and if text, may
be other than XML. Each unparsed entity has an associated notation, identified by name. Beyond a requirement
that an XML processor make the identifiers for the entity and notation available
to the application, XML places no constraints on the contents of unparsed
entities.

Parsed entities are invoked by name using entity references; unparsed entities
by name, given in the value of ENTITY or ENTITIES attributes.

General entities
are entities for use within the document content. In this specification, general
entities are sometimes referred to with the unqualified term entity
when this leads to no ambiguity.Parameter
entities are parsed entities for use within the DTD.
These two types of entities use different forms of reference and are recognized
in different contexts. Furthermore, they occupy different namespaces; a parameter
entity and a general entity with the same name are two distinct entities.

Character and Entity References

A character
reference refers to a specific character in the ISO/IEC 10646 character
set, for example one not directly accessible from available input devices.

Characters referred
to using character references MUST match the production for Char.

If the character reference begins with &#x,
the digits and letters up to the terminating ; provide a hexadecimal
representation of the character's code point in ISO/IEC 10646. If it begins
just with &#, the digits up to the terminating ;
provide a decimal representation of the character's code point.

An entity reference
refers to the content of a named entity.References to parsed general entities use
ampersand (&) and semicolon (;) as delimiters.Parameter-entity references
use percent-sign (%) and semicolon (;) as delimiters.

In a document
without any DTD, a document with only an internal DTD subset which contains
no parameter entity references, or a document with standalone='yes', for
an entity reference that does not occur within the external subset or a parameter
entity, the Name given in the entity reference MUSTmatch that in an entity
declaration that does not occur within the external subset or a
parameter entity, except that well-formed documents need not declare
any of the following entities: &magicents;. The
declaration of a general entity MUST precede any reference to it which appears
in a default value in an attribute-list declaration.

Note
that non-validating processors are not
obligated to to read and process entity declarations occurring in parameter entities or in
the external subset; for such documents,
the rule that an entity must be declared is a well-formedness constraint only
if standalone='yes'.

Entity Declared

In a document with an external subset or external parameter entitiesentity references with
standalone='no',
the Name given in the entity reference MUSTmatch that in an entity
declaration. For interoperability, valid documents SHOULD declare
the entities &magicents;, in the form specified in .
The declaration of a parameter entity MUST precede any reference to it. Similarly,
the declaration of a general entity MUST precede any attribute-list
declaration containing a default value with a direct or indirect reference
to that general entity.

Parsed Entity

An entity reference MUST
NOT contain the name of an unparsed entity.
Unparsed entities may be referred to only in attribute
values declared to be of type ENTITY or ENTITIES.

No Recursion

A parsed entity MUST NOT contain a recursive reference to itself, either directly or indirectly.

In DTD

Parameter-entity references MUST NOT appear outside
the DTD.

Examples of character and entity references:

Type <key>less-than</key> (&#x3C;) to save options.
This document was prepared on &docdate; and
is classified &security-level;.

The Name identifies the entity in an entity
reference or, in the case of an unparsed entity, in the value of
an ENTITY or ENTITIES attribute. If the same entity is declared
more than once, the first declaration encountered is binding; at user option,
an XML processor &MAY; issue a warning if entities are declared multiple times.

Internal Entities

If the
entity definition is an EntityValue, the defined
entity is called an internal entity. There is no separate physical
storage object, and the content of the entity is given in the declaration.
Note that some processing of entity and character references in the literal entity value may be required to produce
the correct replacement text: see .

An internal entity is a parsed entity.

Example of an internal entity declaration:

<!ENTITY Pub-Status "This is a pre-release of the
specification.">
External Entities

If the entity is not internal,
it is an external entity, declared as follows:

If the NDataDecl is present, this is a general unparsed entity; otherwise it is a parsed entity.

Notation Declared

The NameMUST match the declared name of a notation.

The SystemLiteral is called the entity's system
identifier. It is meant to be
converted to a URI reference
(as defined in , updated by ),
as part of the
process of dereferencing it to obtain input for the XML processor to construct the
entity's replacement text. It is an error for a fragment identifier
(beginning with a # character) to be part of a system identifier.
Unless otherwise provided by information outside the scope of this specification
(e.g. a special XML element type defined by a particular DTD, or a processing
instruction defined by a particular application specification), relative URIs
are relative to the location of the resource within which the entity declaration
occurs. This is defined to
be the external entity containing the '<' which starts the declaration, at the
point when it is parsed as a declaration.
A URI might thus be relative to the document
entity, to the entity containing the external
DTD subset, or to some other external parameter
entity. Attempts to
retrieve the resource identified by a URI &may; be redirected at the parser
level (for example, in an entity resolver) or below (at the protocol level,
for example, via an HTTP Location: header). In the absence of additional
information outside the scope of this specification within the resource,
the base URI of a resource is always the URI of the actual resource returned.
In other words, it is the URI of the resource retrieved after all redirection
has occurred.

System
identifiers (and other XML strings meant to be used as URI references) &may; contain
characters that, according to and ,
must be escaped before a URI can be used to retrieve the referenced resource. The
characters to be escaped are the control characters #x0 to #x1F and #x7F (most of
which cannot appear in XML), space #x20, the delimiters '<' #x3C, '>' #x3E and
'"' #x22, the unwise characters '{' #x7B, '}' #x7D, '|' #x7C, '\' #x5C, '^' #x5E and
'`' #x60, as well as all characters above #x7F. Since escaping is not always a fully
reversible process, it MUST be performed only when absolutely necessary and as late
as possible in a processing chain. In particular, neither the process of converting
a relative URI to an absolute one nor the process of passing a URI reference to a
process or software component responsible for dereferencing it SHOULD trigger escaping.
When escaping does occur, it MUST be performed as follows:

Each character to be escaped is represented in
UTF-8
as one or more bytes.

The resulting bytes are escaped with
the URI escaping mechanism (that is, converted to %HH,
where HH is the hexadecimal notation of the byte value).

The original character is replaced by the resulting character sequence.

In addition to a system
identifier, an external identifier &may; include a public identifier.
An XML processor attempting to retrieve the entity's content &may; use
any combination of
the public and system identifiers as well as additional information outside the
scope of this specification to try to generate an alternative URI reference.
If the processor is unable to do so, it MUST use the URI
reference specified in the system literal. Before a match is attempted,
all strings of white space in the public identifier MUST be normalized to
single space characters (#x20), and leading and trailing white space MUST
be removed.

The text declaration MUST be provided literally, not by reference
to a parsed entity. The text declaration
MUST NOT appear at any
position other than the beginning of an external parsed entity. The text declaration
in an external parsed entity is not considered part of its replacement text.

Well-Formed Parsed Entities

The document entity is well-formed if it matches the production labeled document. An external general parsed entity is well-formed
if it matches the production labeled extParsedEnt. All
external parameter entities are well-formed by definition.

Only parsed entities that are referenced directly or indirectly within the document are required to be well-formed.

An internal general parsed entity is well-formed if its replacement text
matches the production labeled content. All internal
parameter entities are well-formed by definition.

A consequence of well-formedness in general
entities is that the logical and physical
structures in an XML document are properly nested; no start-tag, end-tag, empty-element tag, element, comment, processing instruction, character
reference, or entity reference
can begin in one entity and end in another.

Character Encoding in Entities

Each external parsed entity in an XML document &may; use a different encoding
for its characters. All XML processors MUST be able to read entities in both
the UTF-8 and UTF-16 encodings. The terms UTF-8
and UTF-16 in this specification do not apply to character
encodings with any other labels, even if the encodings or labels are very
similar to UTF-8 or UTF-16.

Entities encoded in UTF-16 MUST and entities
encoded in UTF-8 &MAY; begin with the Byte Order Mark described in
ISO/IEC 10646 or Unicode
(the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature,
not part of either the markup or the character data of the XML document. XML
processors MUST be able to use this character to differentiate between UTF-8
and UTF-16 encoded documents.

Although an XML processor is required to read only entities in the UTF-8
and UTF-16 encodings, it is recognized that other encodings are used around
the world, and it may be desired for XML processors to read entities that
use them. In
the absence of external character encoding information (such as MIME headers),
parsed entities which are stored in an encoding other than UTF-8 or UTF-16
MUST begin with a text declaration (see ) containing
an encoding declaration:

In the document entity, the encoding
declaration is part of the XML declaration.
The EncName is the name of the encoding used.

In an encoding declaration, the values UTF-8, UTF-16,
ISO-10646-UCS-2, and ISO-10646-UCS-4SHOULD be used
for the various encodings and transformations of Unicode / ISO/IEC 10646,
the values ISO-8859-1, ISO-8859-2,
... ISO-8859-n (where n
is the part number) SHOULD be used for the parts of ISO 8859, and
the values ISO-2022-JP, Shift_JIS,
and EUC-JPSHOULD be used for the various encoded
forms of JIS X-0208-1997. It
is RECOMMENDED that character encodings registered (as charsets)
with the Internet Assigned Numbers Authority ,
other than those just listed, be referred to using their registered names;
other encodings SHOULD use names starting with an x- prefix.
XML processors SHOULD match character encoding names in a case-insensitive
way and SHOULD either interpret an IANA-registered name as the encoding registered
at IANA for that name or treat it as unknown (processors are, of course, not
required to support all IANA-registered encodings).

In the absence of information provided by an external transport protocol
(e.g. HTTP or MIME), it is a fatal error for
an entity including an encoding declaration to be presented to the XML processor
in an encoding other than that named in the declaration, or for an entity which
begins with neither a Byte Order Mark
nor an encoding declaration to use an encoding other than UTF-8. Note that
since ASCII is a subset of UTF-8, ordinary ASCII entities do not strictly
need an encoding declaration.

It is a fatal error for a TextDecl to occur other
than at the beginning of an external entity.

It is a fatal error when an XML processor
encounters an entity with an encoding that it is unable to process. It
is a fatal error if an XML entity is determined (via default, encoding declaration,
or higher-level protocol) to be in a certain encoding but contains byte
sequences that are not legal in that encoding. Specifically, it is a
fatal error if an entity encoded in UTF-8 contains any irregular code unit sequences,
as defined in Unicode . Unless an encoding
is determined by a higher-level protocol, it is also a fatal error if an XML entity
contains no encoding declaration and its content is not legal UTF-8 or UTF-16.

Each entity, including the document entity,
can be separately
declared as XML 1.0 or XML 1.1. The version declaration appearing
in the document entity determines the version of the document as a
whole. An XML 1.1 document may invoke XML 1.0 external entities, so
that otherwise duplicated versions of external entities,
particularly DTD external subsets, need not be maintained. However,
in such a case the rules of XML 1.1 are applied to the entire
document.

If an entity (including the document entity) is not labeled
with a version number, it is treated as if labeled as version
1.0.

XML Processor Treatment of Entities and References

The table below summarizes the contexts in which character references,
entity references, and invocations of unparsed entities might appear and the
REQUIRED behavior of an XML processor
in each case. The labels in the leftmost column describe the recognition context: Reference in Content

as a reference anywhere after the start-tag
and before the end-tag of an element; corresponds
to the nonterminal content.

Reference in Attribute Value

as a reference within either the value of an attribute in a start-tag,
or a default value in an attribute declaration;
corresponds to the nonterminal AttValue.

Occurs as Attribute Value

as a Name, not a reference, appearing either as
the value of an attribute which has been declared as type ENTITY,
or as one of the space-separated tokens in the value of an attribute which
has been declared as type ENTITIES.

Reference in Entity Value

as a reference within a parameter or internal entity's literal
entity value in the entity's declaration; corresponds to the nonterminal EntityValue.

Reference in DTD

as a reference within either the internal or external subsets of the DTD, but outside of an EntityValue, AttValue, PI, Comment, SystemLiteral, PubidLiteral,
or the contents of an ignored conditional section (see ).

.

Entity
Type

Character

Parameter

Internal General

External Parsed
General

Unparsed

Reference
in Content

Not recognized

Included

Included
if validating

Forbidden

Included

Reference in Attribute Value

Not recognized

Included
in literal

Forbidden

Forbidden

Included

Occurs as Attribute
Value

Not recognized

Forbidden

Forbidden

Notify

Not recognized

Reference in EntityValue

Included in literal

Bypassed

Bypassed

Error

Included

Reference in DTD

Included as PE

Forbidden

Forbidden

Forbidden

Forbidden

Not Recognized

Outside the DTD, the % character has no special significance;
thus, what would be parameter entity references in the DTD are not recognized
as markup in content. Similarly, the names of unparsed
entities are not recognized except when they appear in the value of an appropriately
declared attribute.

Included

An entity is included
when its replacement text is retrieved
and processed, in place of the reference itself, as though it were part of
the document at the location the reference was recognized. The replacement
text &may; contain both character data
and (except for parameter entities) markup,
which MUST be recognized in the usual way. (The string AT&amp;T;
expands to AT&T; and the remaining ampersand
is not recognized as an entity-reference delimiter.) A character reference
is included when the indicated character is processed in place
of the reference itself.

Included If Validating

When an XML processor recognizes a reference to a parsed entity, in order
to validate the document, the processor
MUSTinclude its replacement text. If
the entity is external, and the processor is not attempting to validate the
XML document, the processor &MAY;, but need
not, include the entity's replacement text. If a non-validating processor
does not include the replacement text, it MUST inform the application that
it recognized, but did not read, the entity.

This rule is based on the recognition that the automatic inclusion provided
by the SGML and XML entity mechanism, primarily designed to support modularity
in authoring, is not necessarily appropriate for other applications, in particular
document browsing. Browsers, for example, when encountering an external parsed
entity reference, might choose to provide a visual indication of the entity's
presence and retrieve it for display only on demand.

Forbidden

The following are forbidden, and constitute fatal
errors:

the appearance of a reference to an unparsed
entity, except in the
EntityValue in an entity declaration.

the appearance of any character or general-entity reference in the
DTD except within an EntityValue or AttValue.

a reference to an external entity in an attribute value.

Included in Literal

When an entity reference appears in
an attribute value, or a parameter entity reference appears in a literal entity
value, its replacement textMUST be processed
in place of the reference itself as though it were part of the document at
the location the reference was recognized, except that a single or double
quote character in the replacement text MUST always be treated as a normal data
character and MUST NOT terminate the literal. For example, this is well-formed:

]]>

while this is not:

<!ENTITY EndAttr "27'" >
<element attribute='a-&EndAttr;>
Notify

When the name of an unparsed entity
appears as a token in the value of an attribute of declared type ENTITY
or ENTITIES, a validating processor MUST inform the application of
the system and public
(if any) identifiers for both the entity and its associated notation.

Bypassed

When a general entity reference appears in the EntityValue
in an entity declaration, it MUST be bypassed and left as is.

Included as PE

Just as with external parsed entities, parameter entities need only be included if validating. When a parameter-entity
reference is recognized in the DTD and included, its replacement
textMUST be enlarged by the attachment of one leading and one following
space (#x20) character; the intent is to constrain the replacement text of
parameter entities to contain an integral number of grammatical tokens in
the DTD. This
behavior MUST NOT apply to parameter entity references within entity values;
these are described in .

Error

It is an error for a reference to
an unparsed entity to appear in the EntityValue in an
entity declaration.

Construction of Entity Replacement Text

In discussing the treatment of entities, it is useful to distinguish
two forms of the entity's value.
For an
internal entity, the literal
entity value is the quoted string actually present in the entity declaration,
corresponding to the non-terminal EntityValue.For an external entity, the literal
entity value is the exact text contained in the entity.For an
internal entity, the replacement text
is the content of the entity, after replacement of character references and
parameter-entity references.For
an external entity, the replacement text is the content of the entity,
after stripping the text declaration (leaving any surrounding white space) if there
is one but without any replacement of character references or parameter-entity
references.

The literal entity value as given in an internal entity declaration (EntityValue) &may; contain character, parameter-entity,
and general-entity references. Such references MUST be contained entirely
within the literal entity value. The actual replacement text that is included (or included in literal) as described above
MUST contain the replacement
text of any parameter entities referred to, and MUST contain the character
referred to, in place of any character references in the literal entity value;
however, general-entity references MUST be left as-is, unexpanded. For example,
given the following declarations:

The general-entity reference &rights; would
be expanded should the reference &book; appear
in the document's content or an attribute value.

These simple rules may have complex interactions; for a detailed discussion
of a difficult example, see .

Predefined Entities

Entity and character references &may;
both be used to escape the left angle bracket, ampersand, and
other delimiters. A set of general entities (&magicents;) is specified for
this purpose. Numeric character references &may; also be used; they are expanded
immediately when recognized and MUST be treated as character data, so the
numeric character references &#60; and &#38; &may; be used to escape < and & when they occur
in character data.

All XML processors MUST recognize these entities whether they are declared
or not. For interoperability, valid XML
documents SHOULD declare these entities, like any others, before using them. If
the entities lt or amp are declared, they MUST be
declared as internal entities whose replacement text is a character reference
to the respective
character (less-than sign or ampersand) being escaped; the double
escaping is REQUIRED for these entities so that references to them produce
a well-formed result. If the entities gt, apos,
or quot are declared, they MUST be declared as internal entities
whose replacement text is the single character being escaped (or a character
reference to that character; the double escaping here is OPTIONAL but harmless).
For example:

]]>
Notation Declarations

Notations identify
by name the format of unparsed entities,
the format of elements which bear a notation attribute, or the application
to which a processing instruction is addressed.

Notation declarations
provide a name for the notation, for use in entity and attribute-list declarations
and in attribute specifications, and an external identifier for the notation
which may allow an XML processor or its client application to locate a helper
application capable of processing data in the given notation.

A given NameMUST NOT be declared in more than one notation declaration.

XML processors MUST provide applications with the name and external identifier(s)
of any notation declared and referred to in an attribute value, attribute
definition, or entity declaration. They &MAY; additionally resolve the external
identifier into the system identifier, file
name, or other information needed to allow the application to call a processor
for data in the notation described. (It is not an error, however, for XML
documents to declare and refer to notations for which notation-specific applications
are not available on the system where the XML processor or application is
running.)

Document Entity

The document entity
serves as the root of the entity tree and a starting-point for an XML processor. This specification does
not specify how the document entity is to be located by an XML processor;
unlike other entities, the document entity has no name and might well appear
on a processor input stream without any identification at all.

Conformance
Validating and Non-Validating Processors

Conforming XML processors fall into
two classes: validating and non-validating.

Validating and non-validating processors alike MUST report violations of
this specification's well-formedness constraints in the content of the document entity and any other parsed
entities that they read.

Validating
processorsMUST,
at user option, report violations of the constraints expressed by
the declarations in the DTD, and failures
to fulfill the validity constraints given in this specification.
To accomplish this, validating XML processors MUST read and process the entire
DTD and all external parsed entities referenced in the document.

Non-validating processors are REQUIRED to check only the document
entity, including the entire internal DTD subset, for well-formedness. While they are not required
to check the document for validity, they are REQUIRED to process
all the declarations they read in the internal DTD subset and in any parameter
entity that they read, up to the first reference to a parameter entity that
they do not read; that is to say, they MUST use the information
in those declarations to normalize
attribute values, include the replacement
text of internal entities, and supply default
attribute values. Except when standalone="yes", they
MUST NOTprocessentity
declarations or attribute-list declarations
encountered after a reference to a parameter entity that is not read, since
the entity may have contained overriding declarations; when standalone="yes", processors MUST
process these declarations.

Note
that when processing invalid documents with a non-validating
processor the application may not be presented with consistent
information. For example, several requirements for uniqueness
within the document may not be met, including more than one element
with the same id, duplicate declarations of elements or notations
with the same name, etc. In these cases the behavior of the parser
with respect to reporting such information to the application is
undefined.

XML 1.1 processors MUST be able to process both XML 1.0
and XML 1.1 documents. Programs which generate XML SHOULD
generate XML 1.0, unless one of the specific features of XML 1.1 is required.

Using XML Processors

The behavior of a validating XML processor is highly predictable; it must
read every piece of a document and report all well-formedness and validity
violations. Less is required of a non-validating processor; it need not read
any part of the document other than the document entity. This has two effects
that may be important to users of XML processors:

Certain well-formedness errors, specifically those that require reading
external entities, may fail to be detected by a non-validating processor. Examples
include the constraints entitled Entity Declared, Parsed Entity, and No
Recursion, as well as some of the cases described as forbidden in .

The information passed from the processor to the application may
vary, depending on whether the processor reads parameter and external entities.
For example, a non-validating processor may fail to normalize
attribute values, include the replacement
text of internal entities, or supply default
attribute values, where doing so depends on having read declarations
in external or parameter entities.

For maximum reliability in interoperating between different XML processors,
applications which use non-validating processors SHOULD NOT rely on any behaviors
not required of such processors. Applications which require DTD facilities
not related to validation (such
as the declaration of default attributes and internal entities that are
or may be specified in
external entities)SHOULD use validating XML processors.

Notation

The formal grammar of XML is given in this specification using a simple
Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines
one symbol, in the form

symbol ::= expression

Symbols are written with an initial capital letter if they are the
start symbol of a regular language, otherwise with an initial lowercase
letter. Literal strings are quoted.

Within the expression on the right-hand side of a rule, the following expressions
are used to match strings of one or more characters: #xN

where N is a hexadecimal integer, the expression matches the character
whose number
(code point) in ISO/IEC 10646 is N. The number of leading zeros in the #xN
form is insignificant.

[a-zA-Z], [#xN-#xN]

matches any Char with a value in the range(s) indicated (inclusive).

[abc], [#xN#xN#xN]

matches any Char with a value among the characters
enumerated. Enumerations and ranges can be mixed in one set of brackets.

[^a-z], [^#xN-#xN]

matches any Char with a value outside the range
indicated.

[^abc], [^#xN#xN#xN]

matches any Char with a value not among the characters given. Enumerations
and ranges of forbidden values can be mixed in one set of brackets.

"string"

matches a literal string matching that
given inside the double quotes.

'string'

matches a literal string matching that
given inside the single quotes.

These symbols may be combined to match more complex patterns as follows,
where A and B represent simple expressions: (expression)

expression is treated as a unit and may be combined as described
in this list.

A?

matches A or nothing; optional A.

A B

matches A followed by B. This
operator has higher precedence than alternation; thus A B | C D
is identical to (A B) | (C D).

A | B

matches A or B.

A - B

matches any string that matches A but does not match B.

A+

matches one or more occurrences of A. Concatenation
has higher precedence than alternation; thus A+ | B+ is identical
to (A+) | (B+).

A*

matches zero or more occurrences of A. Concatenation
has higher precedence than alternation; thus A* | B* is identical
to (A*) | (B*).

Other notations used in the productions are: /* ... */

comment.

[ wfc: ... ]

well-formedness constraint; this identifies by name a constraint on well-formed documents associated with a production.

[ vc: ... ]

validity constraint; this identifies by name a constraint on valid
documents associated with a production.

This appendix contains the necessary definitions for character normalization.
For additional background information and examples, see .

Text is said to be
in a Unicode encoding form if it is encoded in
UTF-8, UTF-16 or UTF-32.

Legacy encoding
is taken to mean any character encoding not based on Unicode.

A
normalizing transcoder is a transcoder that converts from a
legacy encoding to a
Unicode encoding form and
ensures that the result is in Unicode Normalization Form C
(see UAX #15 ).

A character escape
is a syntactic device defined in a markup or programming language that allows
one or more of:

expressing syntax-significant characters while disregarding
their significance in the syntax of the language, or

expressing characters not representable in the character encoding
chosen for an instance of the language, or

expressing characters in general, without use of the corresponding
character codes.

Certified text
is text which satisfies at least one of the following conditions:

it has been confirmed through inspection that the text
is in normalized form

the source text-processing component is identified
and is known to produce only normalized text.

Text is, for the purposes of
this specification, Unicode-normalized if it is in a
Unicode encoding form and is in
Unicode Normalization Form C, according to a version of Unicode Standard Annex #15:
Unicode Normalization Forms at least as recent as the
oldest version of the Unicode Standard that contains all the characters
actually present in the text, but no earlier
than version 3.2.

Text is
include-normalized if:

the text is Unicode-normalized
and does not contain any character escapes
or includes whose expansion would
cause the text to become no longer Unicode-normalized;
or

the text is in a legacy encoding and, if it were transcoded
to a Unicode encoding form by a
normalizing transcoder, the resulting
text would satisfy clause 1 above.

A composing character
is a character that is one or both of the following:

the second character in the canonical decomposition mapping of
some primary composite (as defined in D3 of UAX #15 ), or

of non-zero canonical combining class (as defined in Unicode
).

Text is
fully-normalized if:

the text is in a Unicode encoding
form, is include-normalized and
none of the relevant
constructs comprising the text begin with a
composing character or a
character escape representing a
composing character; or

the text is in a legacy encoding and,
if it were transcoded to a Unicode encoding form
by a normalizing transcoder, the resulting text
would satisfy clause 1 above.

Expansion of Entity and Character References

This appendix contains some examples illustrating the sequence of entity-
and character-reference recognition and expansion, as specified in .

If the DTD contains the declaration

An ampersand (&#38;) may be escaped
numerically (&#38;#38;) or with a general entity
(&amp;)." >]]>

then the XML processor will recognize the character references when it
parses the entity declaration, and resolve them before storing the following
string as the value of the entity example:

An ampersand (&) may be escaped
numerically (&#38;) or with a general entity
(&amp;).]]>

A reference in the document to &example;
will cause the text to be reparsed, at which time the start- and end-tags
of the p element will be recognized and the three references will
be recognized and expanded, resulting in a p element with the following
content (all data, no delimiters or markup):

A more complex example will illustrate the rules and their effects fully.
In the following example, the line numbers are solely for reference.

1
2
3
4
5 ' >
6 %xx;
7 ]>
8 This sample shows a &tricky; method.]]>

This produces the following:

in line 4, the reference to character 37 is expanded immediately,
and the parameter entity xx is stored in the symbol
table with the value %zz;. Since the replacement
text is not rescanned, the reference to parameter entity zz
is not recognized. (And it would be an error if it were, since zz
is not yet declared.)

in line 5, the character reference &#60;
is expanded immediately and the parameter entity zz
is stored with the replacement text
<!ENTITY tricky "error-prone">, which is a well-formed entity declaration.

in line 6, the reference to xx is recognized,
and the replacement text of xx (namely %zz;)
is parsed. The reference to zz is recognized in
its turn, and its replacement text
(<!ENTITY tricky "error-prone">)
is parsed. The general entity tricky
has now been declared, with the replacement text error-prone.

in line 8, the reference to the general entity tricky
is recognized, and it is expanded, so the full content of the test
element is the self-describing (and ungrammatical) string This sample
shows a error-prone method.

Deterministic Content Models

As
noted in , it is required that content
models in element type declarations be deterministic. This requirement is for compatibility with SGML (which calls deterministic
content models unambiguous); XML processors built
using SGML systems may flag non-deterministic content models as errors.

For example, the content model ((b, c) | (b, d)) is non-deterministic,
because given an initial b the XML processor
cannot know which b in the model is being matched without looking
ahead to see which element follows the b. In this case, the two references
to b can be collapsed into a single reference, making the model read (b,
(c | d)). An initial b now clearly matches only a single name
in the content model. The processor doesn't need to look ahead to see what follows; either c or d
would be accepted.

More formally: a finite state automaton may be constructed from the content
model using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of
Aho, Sethi, and Ullman . In many such algorithms, a follow
set is constructed for each position in the regular expression (i.e., each
leaf node in the syntax tree for the regular expression); if any position
has a follow set in which more than one following position is labeled with
the same element type name, then the content model is in error and may be
reported as an error.

Algorithms exist which allow many but not all non-deterministic content
models to be reduced automatically to equivalent deterministic models; see
Brüggemann-Klein 1991 .

Autodetection of Character Encodings

The XML encoding declaration functions as an internal label on each entity,
indicating which character encoding is in use. Before an XML processor can
read the internal label, however, it apparently has to know what character
encoding is in use — which is what the internal label is trying to indicate.
In the general case, this is a hopeless situation. It is not entirely hopeless
in XML, however, because XML limits the general case in two ways: each implementation
is assumed to support only a finite set of character encodings, and the XML
encoding declaration is restricted in position and content in order to make
it feasible to autodetect the character encoding in use in each entity in
normal cases. Also, in many cases other sources of information are available
in addition to the XML data stream itself. Two cases may be distinguished,
depending on whether the XML entity is presented to the processor without,
or with, any accompanying (external) information. We consider the first case
first.

Detection Without External Encoding Information

Because each XML entity not accompanied by external
encoding information and not in UTF-8 or UTF-16 encoding must
begin with an XML encoding declaration, in which the first characters must
be '<?xml', any conforming processor can detect, after two
to four octets of input, which of the following cases apply. In reading this
list, it may help to know that in UCS-4, '<' is #x0000003C
and '?' is #x0000003F, and the Byte Order Mark
required of UTF-16 data streams is #xFEFF. The notation
## is used to denote any byte value except that two consecutive
##s cannot be both 00.

With a Byte Order Mark:

00 00 FE
FF

UCS-4, big-endian machine (1234 order)

FF
FE 00 00

UCS-4, little-endian machine (4321 order)

00 00 FF FE

UCS-4, unusual octet order (2143)

FE FF 00 00

UCS-4, unusual octet order (3412)

FE FF ## ##

UTF-16, big-endian

FF FE ## ##

UTF-16, little-endian

EF BB BF

UTF-8

Without a Byte Order Mark:

00 00 00 3C

UCS-4 or other encoding with a 32-bit code unit and ASCII
characters encoded as ASCII values, in respectively big-endian (1234), little-endian
(4321) and two unusual byte orders (2143 and 3412). The encoding declaration
must be read to determine which of UCS-4 or other supported 32-bit encodings
applies.

3C 00 00 00

00 00 3C 00

00 3C 00 00

00 3C 00 3F

UTF-16BE or big-endian ISO-10646-UCS-2
or other encoding with a 16-bit code unit in big-endian order and ASCII characters
encoded as ASCII values (the encoding declaration must be read to determine
which)

3C 00 3F 00

UTF-16LE or little-endian
ISO-10646-UCS-2 or other encoding with a 16-bit code unit in little-endian
order and ASCII characters encoded as ASCII values (the encoding declaration
must be read to determine which)

3C 3F 78 6D

UTF-8, ISO 646, ASCII, some part of ISO 8859, Shift-JIS, EUC, or any other
7-bit, 8-bit, or mixed-width encoding which ensures that the characters of
ASCII have their normal positions, width, and values; the actual encoding
declaration must be read to detect which of these applies, but since all of
these encodings use the same bit patterns for the relevant ASCII characters,
the encoding declaration itself may be read reliably

4C
6F A7 94

EBCDIC (in some flavor; the full encoding declaration
must be read to tell which code page is in use)

Other

UTF-8 without an encoding declaration, or else the data stream is mislabeled
(lacking a required encoding declaration), corrupt, fragmentary, or enclosed
in a wrapper of some kind

In cases above which do not require reading the encoding declaration to
determine the encoding, section 4.3.3 still requires that the encoding declaration,
if present, be read and that the encoding name be checked to match the actual
encoding of the entity. Also, it is possible that new character encodings
will be invented that will make it necessary to use the encoding declaration
to determine the encoding, in cases where this is not required at present.

This level of autodetection is enough to read the XML encoding declaration
and parse the character-encoding identifier, which is still necessary to distinguish
the individual members of each family of encodings (e.g. to tell UTF-8 from
8859, and the parts of 8859 from each other, or to distinguish the specific
EBCDIC code page in use, and so on).

Because the contents of the encoding declaration are restricted to characters
from the ASCII repertoire (however encoded),
a processor can reliably read the entire encoding declaration as soon as it
has detected which family of encodings is in use. Since in practice, all widely
used character encodings fall into one of the categories above, the XML encoding
declaration allows reasonably reliable in-band labeling of character encodings,
even when external sources of information at the operating-system or transport-protocol
level are unreliable. Character encodings such as UTF-7
that make overloaded usage of ASCII-valued bytes may fail to be reliably detected.

Once the processor has detected the character encoding in use, it can act
appropriately, whether by invoking a separate input routine for each case,
or by calling the proper conversion function on each character of input.

Like any self-labeling system, the XML encoding declaration will not work
if any software changes the entity's character set or encoding without updating
the encoding declaration. Implementors of character-encoding routines should
be careful to ensure the accuracy of the internal and external information
used to label the entity.

Priorities in the Presence of External Encoding Information

The second possible case occurs when the XML entity is accompanied by encoding
information, as in some file systems and some network protocols. When multiple
sources of information are available, their relative priority and the preferred
method of handling conflict should be specified as part of the higher-level
protocol used to deliver XML. In particular, please refer
to or its successor, which defines the text/xml
and application/xml MIME types and provides some useful guidance.
In the interests of interoperability, however, the following rule is recommended.

If an XML entity is in a file, the Byte-Order Mark and encoding declaration are used
(if present) to determine the character encoding.

W3C XML Working Group

This specification was prepared and approved for publication by the W3C
XML Working Group (WG). WG approval of this specification does not necessarily
imply that all WG participants voted for its approval. The current and former members
in the XML WG are:

This edition was encoded in a
slightly modified version of the
XMLspec DTD, 2.10.
The XHTML versions were produced with a combination of the
xmlspec.xsl,
diffspec.xsl,
and REC-xml.xsl
XSLT stylesheets.

Suggestions for XML Names

The following suggestions define what is believed to be best
practice in the construction of XML names used as element names,
attribute names, processing instruction targets, entity names,
notation names, and the values of attributes of type ID, and are
intended as guidance for document authors and schema designers.
All references to Unicode are understood with respect to
a particular version of the Unicode Standard greater than or equal
to 3.0; which version should be used is left to the discretion of
the document author or schema designer.

The first two suggestions are directly derived from the rules
given for identifiers in the Unicode Standard, version 3.0, and
exclude all control characters, enclosing nonspacing marks,
non-decimal numbers, private-use characters, punctuation characters
(with the noted exceptions), symbol characters, unassigned
codepoints, and white space characters. The other suggestions
are mostly derived from Appendix B.

The first character of any name should have a Unicode General
Category of Ll, Lu, Lo, Lm, Lt, or Nl, or else be '_' #x5F.

Characters other than the first should have a Unicode General
Category of Ll, Lu, Lo, Lm, Lt, Mc, Mn, Nl, Nd, Pc, or Cf, or else
be one of the following: '-' #x2D, '.' #x2E, ':' #x3A or
'·' #xB7 (middle dot). Since Cf characters are not
directly visible, they should be employed with caution and only
when necessary, to avoid creating names which are distinct to XML
processors but look the same to human beings.

Ideographic characters which have a canonical decomposition
(including those in the ranges [#xF900-#xFAFF] and
[#x2F800-#x2FFFD], with 12 exceptions) should not be used in names.

Characters which have a compatibility decomposition (those with
a "compatibility formatting tag" in field 5 of the Unicode
Character Database -- marked by field 5 beginning with a "<")
should not be used in names. This suggestion does not apply
to #x0E33 THAI CHARACTER SARA AM or #x0EB3 LAO CHARACTER AM, which
despite their compatibility decompositions are in regular use in
those scripts.

Combining characters meant for use with symbols only (including
those in the ranges [#x20D0-#x20EF] and [#x1D165-#x1D1AD]) should
not be used in names.

The interlinear annotation characters ([#xFFF9-#xFFFB]) should
not be used in names.

Variation selector characters should not be used in names.

Names which are nonsensical, unpronounceable, hard to read, or
easily confusable with other names should not be employed.